Human infections of the oral and pleural cavity, bloodstream, heart, and other organs due to Bacteroides species are being more frequently recognized. Definitive bacterological diagnosis is difficult and slow due to the oxygen sensitivity and growth characteristics of the bacteria involved. In similar situations the detection of specific bacterial antigens in patient materials has permitted an early identification of the etiologic agent and the immediate institution of proper therapy. In this project we will examine clarified tissue abscess fluids and serum for the presence of Bacteroides antigens by counterimmunoelectrophoresis and passive hemagglutination with hyperimmune rabbit antisera prepared against several Bacteroides species including B. fragilis and B. melaninogenicus. Parallel bacteriological studies will indicate the suitability of antigen detection as a diagnostic aid as will experiments with model infections in animals.